Digital-to-analog converters (DACs) play a critical role in transforming information from a digital domain into an analog domain. DACs generally convert an abstract finite precision number (such as a fixed-point binary number) into a physical quantity (such as a voltage or current). The number of output levels for a given DAC generally corresponds to its resolution, which is based on the number of “bits” of a given digital code that defines each output level. A variety of DAC architectures exist, including voltage-mode schemes and current steering schemes.
FIG. 1 illustrates a conventional voltage-mode DAC, generally designated 100. The DAC includes a data input 102 that receives a data word of multiple bits during a bit time and a clock input 104 that receives a clock. Respective supply (VDD) and ground (VSS) reference voltages are provided to the DAC, and define two switchable states for the output. Multiple DAC output units include switch circuitry (not shown) that responds to the clock to generate corresponding outputs that are weighted and summed to create an overall output voltage level corresponding to the input word code, but in the analog domain.
FIG. 2 illustrates a conventional two-state output unit grouped with other units to create an array for the DAC architecture of FIG. 1. The output unit includes an output impedance ROUT, and switches SW1 and SW2 to switch either the high or low reference voltage across the impedance to an output node. While the two-state approach provides acceptable linearity characteristics, it is less power-efficient.
One solution to the power-efficiency issue of conventional two-state output units is addressed with conventional tri-state output units, such as shown in FIG. 3. Tri-state output units employ a third intermediate voltage state that may be switched-in to reduce power dissipation. Although less power hungry, the tri-state approach is not as linear as the two-state approach.
What is needed is a flexible method and apparatus to support DAC operations for a voltage-mode DAC driver.